The present invention relates to a scrambling type video transmitting/receiving method and an apparatus therefore.
A conventional signal processing system for digital television (TV) broadcasting has a video transmitter and a video receiver. The video transmitter has a Codec (Coder-Decoder) (compression) including a scan conversion circuit. An original video signal or picture input to the Codec is subjected to scan conversion by the scan conversion circuit, subjected to DCT (Discrete Cosine Transform) by a DCT circuit, quantized by a quantizing circuit, controlled by a QSC circuit as to the amount of code, and then subjected to variable-length coding by a VLC (Variable-Length Coding) circuit. The resulting compressed video signal is scrambled by a scrambling circuit. An ECC (Error Correcting Code) is added to the scrambled video signal by an ECC adding circuit. The scrambled video with the ECC is modulated by a modulating circuit to turn out a signal to be transmitted.
The video receiver includes a demodulating circuit for demodulating the signal received from the video transmitter. The demodulated signal is subjected to ECC processing by an ECC processing circuit and then correctly descrambled by a descrambling circuit to turn out a descrambled compressed video signal. The descrambled compressed video signal is subjected to variable-length decoding by a VLD circuit included in a Codec (expansion), dequantized by a dequanting circuit, subjected to IDCT (Inverse DCT) by an IDCT circuit, and then subjected to inverse scan conversion by an inverse scan converting circuit, whereby a picture is reproduced.
Various methods for scrambling the compressed video signal have been proposed in the past in relation to the Codec (compression). The simplest method is to generate random data by use of a particular generator polynomial and add the random data to video/audio data to be transmitted by use of modulo 2 addition. For example, assume a generator polynomial: EQU g(X)=X.sup.15 +X+1 Eq. (1)
Then, random data are produced from preset data which are all "1".
Each subscriber is provided with the descrambling circuit of the Codec (expansion) based on the above Eq. (1) and is allowed to obtain correct video/audio data. Persons other than subscribers are unable to see the Eq. (1) and obtain only the scrambled meaningless data. Scrambling meaningless data will be referred to as scrambling hardy or "fully (or strongly) scrambling" hereinafter. In this sense, the conventional scrambling circuit scrambles the compressed image data hard (fully or strongly).
Technologies relating to the present invention are taught in, e.g., Japanese Patent Laid-Open Publication No. 7-111647 (document 1 hereinafter). Document 1 relates to a scrambler constructed to allow only a limited person to reproduce video data, i.e., a signal processing device capable of executing scrambling and descrambling which controls the appearance of a picture in accordance with a desired purpose. Document 1 uses an original signal in the form of a video signal based on MPEG (Motion Picture Expert Group) standards. A code detector reads the individual code, detects a "det type" signal, and delivers the detected signal to a random number generator. The random number generator generates random numbers on the basis of a scramble key. An Ex-OR (Exclusive OR) gate produces an Ex-OR of a signal output from the random number generator and the original signal, thereby outputting a scrambled signal. The reproducing side performs the above processing in the inverse order with the same configuration. With this construction, it is possible to achieve not only control over the scrambling effect, but also a desired special effect.
Japanese Patent Laid-Open Publication No. 7-67096 (document 2 hereinafter) pertains to a scrambler constructed to protect a reproducing unit from malfunction ascribable to the coincidence of scrambled data with a reservation word, thereby limiting a person expected to reproduce video data. A code detector reads an input signal as to image data based on the MPEG standard, selects a part of the signal which will not coincide with a reservation word as a signal to be scrambled, and then delivers a scramble command to a random number generator. The random number generator generates random numbers only when it receives the scramble command from the code detector, and produces an Ex-OR. The reproducing side performs the above procedure in the inverse order.
Further, Japanese Patent Laid-Open Publication No. 4-8029 (document 3 hereinafter) proposes an enciphering coder capable of enciphering only the initial value of a coding system by use of an encipher key and sending the enciphered value. Document 3 contemplates to enhance secrecy, to lower the processing speed of an enciphering section, to scale down hardware, and to reduce the cost. In document 3, data compressing means is implemented as a compressor for compressing an eight-bit digital signal to a four-bit code. A signal converted by signal converting means is compressed by a coding system and then sent. Enciphering means enciphers only the initial value by using a sixty-four-bit key agreed beforehand only between a transmitting person and a receiving person. In the case of TV, the Initial value is given every scanning line or every segment of a single scanning line, so that enciphering only the initial value ensures secrecy.
Moreover, Japanese Patent Laid-Open Publication No. 5-145923 (document 4 hereinafter) teaches a replaceable secrecy protection module. In document 4, a program signal is scrambled by a key. The key is enciphered twice and then multiplexed with the scrambled program signal. A decoder performs first key decoding by using a signal sequence stored therein and assigned to second secrecy. Then, the replaceable secrecy protection module performs decoding by using a number sequence stored therein and assigned to first secrecy. Subsequently, the decoder descrambles the program by using a twice decoded key.
As stated above, the conventional scrambling methods each scramble a video signal hard (fully or strongly), so that the resulting data are entirely meaningless for persons other than subscribers. However, it has been reported that data allowing an unauthorized person to recognize at least the outline of a picture, as distinguished from the entirely meaningless random data, encourage the person to make subscription with the intention of enjoying complete video and/or audio data. In fact, as for analog broadcasting, WOWOW (one of Japanese broadcasting stations) is broadcasting video whose outlines can be barely recognized by unauthorized persons and has successfully increased the number of subscribers.
Documents 1 and 2 each scrambles data by use of a random number generator and therefore outputs entirely meaningless scrambled data like the traditional scrambling method. Document 3 enciphers only the initial value of coded data by using an encipher key and differs from the scrambling technology. Document 4 pertains to the scrambling of a program signal by use of a key and entirely differs from the present invention as to the kind of data to be scrambled.